The NIH Tetramer Facility provides custom synthesis and distribution of soluble major histocompatibility complex (MHC)-peptide tetramer reagents that can be used to detect antigen-specific T cells. These reagents include custom class I tetramers for mouse, non-human primate, and human alleles;class II reagents for mouse, non-human primate and human alleles;mouse and human CD1d tetramers;and human CD1a-c tetramers. The NIH Tetramer Facility also is developing novel technologies to improve production and expand the range of available MHC and CD1 tetramers. The tetramer reagents can be applied to studies ranging from basic immunology and protection against microbial pathogens to control of immune-mediated diseases and tumor metastases.